1. Field of the Invention
The present invention relates to a securing method of a polymer insulator. More specifically, the present invention relates to a method of inserting a core into a depression portion of a metal member having an open end, and connecting the metal member to the core by circumferentially compressing the core and the metal member by means of a compression die. The present invention also relates to the die used for this securing method.
2. Related Art Statement
Generally, various securing methods for inserting a core into a depression portion of a metal member having an open end and connecting the metal member to the core by compressing circumferentially the core and the metal member by means of a compression die known. For example, Japanese Patent Publication No. 60-54730 (JP-B-60-54730) discloses, such a technique. As shown in FIG. 7, an FRP core 51 and a metal member 52 are connected by performing a compression operation such that a compression pressure P is circumferentially applied at once to a compression portion L of the FRP core 51 to the metal member 52 to be compressed using a compression die 53 having a width corresponding to the compression portion L. In this technique, since the metal member 52 is plastically deformed in a direction vertical to the applied compression pressure, a tensile pressure is applied to the FRP core 51 in its axial direction. Moreover, the compression pressure is simultaneously applied to the FRP core 51 in its radial direction. The resultant polymer insulator formed by the above securing method exhibits decreased strength over time due to a load-time property. This is because the FRP core 51 shows a creep. The strength decreasing rate is defined in the IEC standard, which is theoretically satisfied by the polymer insulator formed by the above securing method. However, it is desirable that the long-term strength decreasing rate deteriorates as slowly as possible to ensure that the polymer insulator is safer when used in an actual line.
Moreover, in the polymer insulator manufactured by the known securing method, a tensile stress is applied, a stress concentration occurs near the end of a compression portion L on the open end of metal member 52. When this stress is applied for a long time, the FRP core 51 will fracture. Therefore, it is necessary to eliminate the stress concentration mentioned above so as to decrease the long-term strength decreasing rate.